Recent, more stringent government regulation of heavy duty diesel engine tailpipe emissions has required new means to reduce these emissions. These regulations cover both new vehicles and existing vehicles. The two most common emissions control devices are the catalytic converter and the carbon particulate trap. Both devices contain and utilize a honeycomb monolith as the active component.
Internally, catalytic converters contain a channeled, ceramic monolith, through which the exhaust gas passes. Viewed from the entrance port, the ceramic monolith resembles a honeycomb structure and may have as many as 400 entrance cells (channels) per square inch. The surface area along the length of these channels is carefully impregnated with precious metals and is extremely porous to increase contact surface area. The precious metals create a catalytic reaction when combined with heat, excess oxygen and noxious exhaust gasses. The catalytic reaction converts unburned hydrocarbon, carbon monoxide and oxides of nitrogen into non-toxic carbon dioxide and water. In addition to creating a catalytic reaction with exhaust gasses, catalytic converters also collect carbon particulate matter (black soot). Catalytic converters create heat during the catalytic reaction that can burn a portion of carbon particulate matter as the exhaust passes through the monolith structure.
Particulate traps are similar to catalysts, as they both use a ceramic monolith as the basic component, but traps are designed only to collect carbon particulate matter in diesel exhaust. Typically, the exhaust channels along the surface of the trap monolith are configured with carefully designed obstructions. This special ceramic monolith surface collects a high percentage of suspended carbon particles and quickly coats with black soot. Trap-type ceramic monoliths have additionally been coated with precious metals, which may only temporarily behave as a reduction catalyst as they quickly become coated with carbon material and need to be cleaned. To this point, however, no device or system has been presented which performs primarily as a reduction catalyst for heavy-duty diesel engine exhaust.
Prior on-board, self-cleaning carbon traps, such as disclosed in U.S. Pat. No. 4,544,388, quickly become filled with particulate matter and need to be routinely regenerated (cleaned) to maintain a level of efficiency. Heretofore, cleaning has required complicated and expensive on-board systems contributing to both complexity and cost. To regenerate a trap device, intense heat must be applied to initiate the ignition and chain reaction combustion of the accumulated carbon. The most common and practical heating methods use on-board electric resistance heaters as disclosed in U.S. Pat. No. 4,516,993. Unfortunately, there is a high electric current requirement to produce sufficient heat to start the regeneration process. This amount of power is not readily available on motor vehicles and, typically, this power deficiency requires complex means to sequence, control or ration electrical consumption. In addition, these on-board systems require the engines to be running to utilize some part of the vehicle exhaust.
Another problem is that after ignition, during regeneration, prior art devices depend upon burning particulate on the surface of the ceramic monolith to generate additional heat to carry out the process fully. There is a high likelihood of overheating during this chain reaction which can cause damage to the monolith. Also, complete regeneration must be attained by a single ignition because there is no provision for re-ignition after regeneration has begun. Furthermore, exhaust bypass valves and separate means to bring oxygen-rich air into the process are required. Due to the complexity and cost associated with this approach, it tends to be problematic and not cost effective for retrofit application.
Other prior devices locate electric resistance heaters in close proximity or directly attached to the surface of the monolith as disclosed in U.S. Pat. No. 4,456,457. Application of heat to specific locations on the monolith face will provide particulate ignition points at desirable multiple points across the face and then down its length. This type of system is disclosed in U.S. Pat. No. 4,331,454 in which a short initial burst or sequence of short bursts of heat light off the face on specific points in the catalyst. This is done with the engine running. In other prior art systems, such as disclosed in U.S. Pat. No. 4,686,827, after initial carbon light-off, re-combustion continues to burn without any further assistance from a heater. Hence, the carbon tends to burn hot and fast without direct control, while migrating around and along the catalyst. To protect against overheating, elaborate heat defusers have been used to protect the ceramic monolith from intense radiant heat from electric heat elements. In addition, deflectors as disclosed in U.S. Pat. No. 4,276,066 have been used to carefully aim the heat in an attempt to create a burn pattern least likely to result in overheating the monolith. The above-mentioned patents represent the closest prior art of which the applicant is aware.
Reducing diesel engine emissions with catalytic converters and particulate traps is well established, however, clogging and cleaning difficulties due to collection of carbon particles on the monolith have prevented the wide use of carbon traps and, in particular, prevented use of emission-reduction catalytic converters with heavy-duty diesel engines. Although self-cleaning particulate traps have been tried, no prior device is known which successfully cleans the particulate build-up on Catalytic converters of diesel engine exhaust. No device is known that will regenerate the ceramic monolith structures of catalysts or traps from a central service point utilizing heated air as the igniter, controlled from an external source without assistance from the motor.
There is therefore a need in the internal combustion exhaust emission arts for cleaning a diesel engine exhaust filtration system to reduce emissions which: is not limited by available vehicle power; is inexpensive to produce, install and operate; will not overheat or produce any undesirable side effects during operation; and can inexpensively clean an exhaust catalyst or trap through complete regeneration at maintenance intervals as desired.